DE69119708T2 - Longitudinal mode selection laser - Google Patents

Description

Background of the invention

The invention relates to lasers and in particular relates to a laser doped with an element from the group of rare earths with longitudinal mode selection. The doped laser is implemented in an optical transmission medium, for example in an optical fiber or a silica waveguide. The rare earth material can comprise, for example, erbium or neodymium.

Applications of fiber optic technology are expanding rapidly. Telecommunications, sensor technology, medical and video transmissions: all of these There are many areas where optical technology can be used, particularly where virtually unlimited bandwidth and low attenuation are important. Cable television systems are one example where fiber optic technology provides efficient and economical alternatives to earlier coaxial cable distribution systems.

Many applications of fiber optics could be made more cost effective if more powerful lasers in the 1.3 or 1.5 micrometer (µm) range were available. It has been demonstrated that an Er³⁺ fiber laser can deliver high power levels. See, for example, MS O'Sullivan et al.: "High Power Narrow Linewidth Erbium- Doped Fiber Laser", CLEO 1989, TUP3, pp. 134-136. A disadvantage of an erbium-doped fiber laser as described in the cited article is that it is longitudinally multimode. Some applications, such as the transmission of complex video signals over an optical fiber in a cable television distribution network or the like, require that the laser operate with only a single longitudinal mode, or at most with only a few such modes sufficiently spaced apart in optical frequency. Otherwise, beats will occur (eg between optical longitudinal modes falling in the high frequency domain), resulting in unacceptable noise levels in the RF band of interest. Those modes of laser operation that are sufficiently close to each other to be in the order of RF frequencies (ie 5.75 MHz to 550 MHz or higher for the cable TV spectrum) must be suppressed to allow such lasers to be used for CATV (cable television) applications. After suppression, the remaining modes alone will not produce beat frequencies in the RF domain. Longitudinal multimode operation may also exhibit excess intensity noise (RIN) due to mode partition noise as the various modes turn on and off. As is well known in the art, the actual modes present in a given laser are determined by the geometry of the laser cavity, the laser gain spectrum, and frequency selective elements in the cavity.

From ECOC 87, Technical Digest, Vol III, Post Deadline Papers, pp. 89-94, a short rare-earth-doped fiber laser is known, which includes an etched reflection fiber grating to introduce wavelength-selective feedback.

It would be advantageous to create a rare earth element-doped laser in which all but one mode are suppressed. Those skilled in the art will appreciate that other modes may still be present as long as they are sufficiently spaced apart in frequency so that no beats occur. Such a laser should be able to operate without generating spurious beats in the RF spectrum. The resulting energy from the laser must comprise a well-defined optical carrier for signal transmission.

The present invention provides a laser having the advantages mentioned above.

Summary of the invention

According to the invention there is provided a laser as defined in claim 1 for providing a well-defined optical carrier. The laser comprises an optical transmission medium, e.g. an optical fiber, with a laser cavity having a portion doped with a rare earth element. Connected to the cavity is means for suppressing at least one mode thereof. The suppressing means comprises an optical grating within the transmission medium. The suppressing means may be provided in the doped portion of the cavity or outside the doped portion.

In one embodiment, the transmission medium has a ring configuration and the suppressing means comprises an optical grating within a portion of the ring. An optical isolator is provided within the ring so that oscillation occurs in only one direction.

According to the invention, a transmission grating is provided within a Fabry-Perot cavity resonator. The Fabry-Perot cavity resonator has a length which is selected is that the longitudinal modes of the laser cavity are confined, and the Moire grating has a transmission peak at a desired one of these modes.

Short description of the characters

Fig. 1 is a schematic representation of an erbium fiber ring laser not according to the invention with a Fabry-Perot interferometer for longitudinal mode selection;

Fig. 2 shows a graph illustrating the pass function of two Fabry-Perot cavity resonators connected in series as in the laser of Fig. 1;

Fig. 3 is a schematic representation of an erbium fiber ring laser according to the invention with a fiber grating for mode selection;

Fig. 4 is a schematic representation of a linear erbium fiber laser not according to the invention with a fiber grating for mode selection;

Fig. 5 is a schematic representation of a linear erbium fiber laser not according to the invention with an external grating for mode selection;

Fig. 6 is a schematic representation of an erbium fiber ring laser according to the invention with series-connected Fabry-Perot cavity resonators and a fiber grating for mode selection;

Fig. 7 is a schematic representation of an erbium fiber laser not according to the invention having a linear combined cavity resonator with a pair of fiber gratings;

Fig. 8 is a schematic representation of a fiber laser not according to the invention with a linear combined cavity resonator, which has a first Fabry-Perot cavity resonator in which a second Fabry-Perot cavity resonator is arranged centrally; and

Fig. 9 is a schematic representation of a fiber laser according to the invention with a linear combined cavity resonator, which has a Fabry-Perot cavity resonator in which a moiré grating is arranged in the center.

Detailed description of the invention

The present invention provides a laser with longitudinal mode selection. Various embodiments are shown. Further embodiments will become apparent to those skilled in the art from the following description.

In the device shown in Fig. 1, a ring cavity laser comprises an erbium-doped optical fiber 14. The erbium dopant (gain medium) provides the optical gain. Erbium is particularly useful for lasers operating in the 1.5 µm range, while neodymium and praseodymium are particularly suitable for the 1.3 µm range. The laser cavity 14 is pumped by a pump laser 10 which is connected by an optical fiber 12 to a conventional coupler 26 which couples the laser cavity to an output fiber 28. For the erbium embodiment, the pump laser 10 can operate at a standard wavelength of 980 nm or 1480 nm. An optical isolator 16 is provided within the ring so that the ring supports traveling wave oscillation in only one direction. The optical isolator is of a commercially available type which only allows one polarization to pass, so that unwanted polarization modes are prevented from lasing. The use of a single polarization allows the realization of a cavity resonator of high finesse. Birefringence is not a problem since only one polarization is excited. The output of the optical Isolator 16 is coupled by an optical fiber 18 to a first mirror 20, which in turn is coupled by an optical fiber 22 to a second mirror 24. The use of mirrors 20 and 24 results in a pair of Fabry-Perot interferometers of unequal length connected in series, which provide wavelength selectivity within the ring laser. A discussion of the operation of Fabry-Perot interferometers can be found in DR Huber and JB Carroll: "Time Domain Response Of An Optically Frequency Swept Fabry-Perot Interferometer", Applied Optics, 1986, Vol. 25, pp. 2386-2390.

In the structure illustrated in Fig. 1, the chaining of Fabry-Perot interferometers within the ring is used to select a smaller number of longitudinal modes within the laser cavity. The lengths of the cavity resonators are chosen to reduce the number of modes. In an implementation with an erbium-doped silicon dioxide substrate, the two cavity resonators can be dimensioned to support only a single mode. According to the invention, a single mode can also be obtained with a single cavity resonator, for example as illustrated in Fig. 3 explained below.

It is desirable to make the diameter of the ring as small as possible. This is due to the fact that the smaller the ring diameter, the further apart the longitudinal modes. Of course, the entire ring can be doped with erbium, or only a region 14 can be doped, as shown in Fig. 1.

Fig. 2 illustrates the pass function, designated overall by "30", of the two cascaded Fabry-Perot resonator cavities in Fig. 1. Coincident frequencies of the coupled resonator cavities add together, and the pass function is the product of the frequencies within each resonator cavity. Thus, for example, by configuring the resonator cavity formed by erbium-doped fiber 14, optical isolator 16, and optical fiber 18 to be resonant at one set of frequencies, and configuring the resonator cavity comprising optical fiber 22 to be resonant at another set of frequencies, only the coincident frequencies 32, 34 in each resonator cavity add together, while all others are essentially cancelled.

Fig. 3 shows an embodiment in which a grating structure 40 replaces the Fabry-Perot interferometer of Fig. 1 in order to obtain mode selection. The functioning of such gratings is discussed in K. L. Belsley, J. B. Carroll, L. A. Hess, D. R. Huber and D. Schmadel: "Optically Multiplexed Interferometric Fiber Optic Sensor System", SPIE, 1985, Vol 566, pp. 257-264 and in W. V. Sorin and S. A. Newton: "Single-Frequency Output From a Broadband-Tunable External-Fiber-Cavity-Laser", OFC 1988, Vol WQ26, p. 123.

To fabricate a laser according to the present invention, the grating dimensions must be chosen to obtain a single-mode frequency peak at a desired wavelength within the gain curve of the erbium laser.

Fig. 4 shows a linear structure that also uses a grating to obtain mode selection. The pump laser 10 provides the optical energy required to lase an erbium-doped fiber 53. The optical fiber 53 is coupled to the pump laser 10. A grating 54 in conjunction with a mirror 52 defines the length of the laser cavity. The dimensions of the grating 54 provide the desired single mode within the gain curve of the laser. An optical isolator 58 limits spurious back reflection into the laser cavity. The optical carrier produced by the laser is output to an optical fiber 60. The length of the laser cavity defines the natural resonance(s) of the cavity. The grid is used to select from these natural resonances.

The linear cavity resonator of the embodiment of Fig. 4 can be made from a piece of erbium-doped optical fiber that includes a grating within the fiber. Alternatively, the linear cavity resonator can include a piece of undoped grating fiber coupled to an erbium-doped fiber. The mirror 52 can for example, by evaporating silver, gold or aluminum at the fiber end and then splicing the fiber. Alternatively, a dielectric mirror could be formed, as is known from the prior art.

The configuration shown in Fig. 4 is one method of achieving lasing at a single frequency. The length of the laser cavity, as determined by reflectors 54 and 52, is short enough so that only one Fabry-Perot mode exists within the cavity. This forces the laser to operate at a single frequency. A specific example of the implementation of such a laser is described below.

The laser can only perform its laser activity within the width of the reflector. A typical reflector bandwidth is 1 angstrom. The mode spacing FSR (free spectral range) of a Fabry-Perot cavity resonator is calculated from the formula

FSR = λ²/(2nL)

wherein

c = speed of light

n = refractive index of the fiber; 1.46

L = laser cavity length

λ= laser wavelength in air; 1.55 µm

Thus, for a mode spacing (FSR) of 1 angstrom, the laser cavity length (L) is 8.2 mm. It is clear that this length is too short for a doped fiber in practical applications. A planar silicon dioxide waveguide highly doped with erbium and germanium could be used to produce a micro-laser.

If a grating bandwidth of one picometer were achieved, then the laser cavity length would be L = 82 cm. In this case, a highly doped erbium fiber used in an 80 cm laser cavity would provide a single frequency laser.

One method of obtaining such a narrow grating is demonstrated in Ragdale, Catherine M. et al.: "Narrow Band Fiber Grating Filters", IEEE Journal on Selected Areas in Communications, Vol. 8, No. 6, August 1990, pp. 1146-1150. An alternative method is shown in the laser combined cavity resonator design of Fig. 7. In this design, gratings 100 and 102 each have a reflection bandwidth of 1 angstrom and reflect at the same wavelength. Since the free spectral range of the Fabry-Perot interferometer formed by the gratings is less than the reflection bandwidth of the gratings, only one Fabry-Perot mode is present within the reflection bandwidth of the gratings. The cavity resonator formed by the grating 100 and the mirror 104 has a length of 80 cm. This provides a FSR of 128 MHz. The FSR for the 8 mm cavity resonator formed by the gratings 100, 102 is 12.8 GHz.

The short cavity resonator in Fig. 7 produces interference phenomena analogous to peaks 32 and 34 in Fig. 2. The more closely spaced interference phenomena in Fig. 2 are analogous to the interference phenomena produced by the 80 cm cavity resonator. In the Fabry-Perot interferometer formed by gratings 100, 102, a finesse of the order of 100 may be required to ensure correct mode selection. It is obvious that the narrower the gratings 100, 102 are in terms of their bandwidth, the easier it is to realize a laser that selects only a single mode.

The simple configuration shown in Fig. 4 can operate at a single frequency even if the optical bandwidth of grating 54 supports more than one Fabry-Perot mode of the cavity formed by reflectors 54 and 52. This is because the erbium laser is predominantly homogeneously broadened so that it attempts to self-select to single frequency oscillation. The combined cavity design described above is also applicable to other laser systems. Neodymium for operation at 1.3 µm is a good example.

Pump lasers that can be used in the practice of the present invention are commercially available from various sources. Gratings are also commercially available, for example from United Technologies Corporation, which incorporates gratings into fibers doped with germanium.

This is accomplished by creating an interference pattern with a high power laser, e.g. at 248 nanometers. This interference pattern is used to expose the fiber, thereby locally modifying the refractive index of the fiber. A similar method is described in the article by Belsley et al. cited above.

Reflectors such as reflector 52 of Figure 4 may comprise partially reflecting mirrors, for example, obtained by splitting the fiber, applying a dielectric coating thereto, and then splicing the fiber back together. Other types of suitable reflectors are commercially available. Substantially lossless mirrors can be obtained, for mirrors having a reflectivity in the range of 5% to 95%. Optical isolators and couplers for use in conjunction with the present invention are also readily available.

Most of the embodiment of an optical arrangement shown in Fig. 5 is applicable in developing the embodiments of the other figures. The pump laser 10 outputs energy to an optical fiber 70 to a coupler 72. A grating lens 74 is coupled to an erbium-doped fiber 76 which receives the pump laser energy via the coupler 72. This coupler should be a wavelength selective coupler to obtain efficient coupling of pump light into the laser cavity and to prevent light at the frequency at which lasing occurs from coupling back into the pump laser. A mirror 78 defines the length of the erbium fiber laser cavity resonator. An optical isolator 80 limits back reflection into the fiber laser. Different grating lenses 74 can be coupled to the erbium fiber in order to empirically obtain the dimensions required for a desired application.

Fig. 6 shows another ring laser design in which the Fabry-Perot interferometers of Fig. 1 have been combined with the grating of Fig. 3. The dimensions of a grating 90 are selected in conjunction with the cavity lengths to obtain operation in a few longitudinal modes or even in only a single longitudinal mode if necessary. The carrier generated by the laser is output via coupler 26 to an optical fiber 92. As in the other embodiments, the position of the mirrors 20, 24 defines a cavity length with desired natural resonances.

It should be noted that in the ring laser configurations shown in Figures 1, 3 and 6, the polarization sensitivity of the standard couplers, grating(s) and optical isolator(s) may make it necessary to introduce polarization control into the ring cavity resonator to increase finesse. Since manual polarization control is not practical for the majority of applications, the solution to the problems of low finesse and polarization sensitivity (birefringence) is to manufacture the fiber laser with a polarization-maintaining fiber and only to excite one of the polarization modes. High finesse couplers that can be used in such embodiments are available from various suppliers.

Fig. 8 shows a laser with a pair of Fabry-Perot cavities coupled in series. A first Fabry-Perot cavity, which is identical to the laser cavity, extends over a length L from a grating 110 to a reflector 116. A second Fabry-Perot cavity is arranged in or near the center of the first Fabry-Perot cavity. The second Fabry-Perot cavity has a length l and is formed by a first reflector 112 and a second reflector 114. The reflectors 112, 114 can be formed within the waveguide, whereby any combination of metallic, dielectric or grating structures known from the prior art can be used. As an alternative, the second Fabry-Perot cavity can be replaced by a moiré grating having a transmission peak at the wavelength at which lasing occurs, such gratings being discussed in the above-referenced article by Ragdale et al. This embodiment is shown in Figure 9, which is similar to the structure shown in Figure 8, except that the positions of the grating 110 and mirror 116 have been arbitrarily reversed and the reflectors 112, 114 have been replaced by a moiré grating 130.

For example, in the configuration shown in Figure 8, grating 110 may have a reflectivity of about 95% at the operating wavelength (e.g., 1.5 µm) of the laser. At this wavelength, reflector 116 will have a reflectivity of about 4 to 15%. Of course, the positions of grating 110 and reflector 116 defining the first Fabry-Perot cavity are interchangeable. In such an embodiment, the reflectivity of reflector 116 adjacent to pump laser 10 would be about 100% at the operating wavelength of the laser, and its transmittance would be about 100% at the pump wavelength. The grating 110, when positioned near the optical isolator 118, would have a reflectivity of approximately 4 to 15% at the operating wavelength of the laser.

In both configurations, the length L of the first Fabry-Perot cavity can be in the order of 1 meter, with the length l of the second Fabry-Perot cavity being in the order of 1 millimeter. The free spectral range of the second Fabry-Perot cavity is slightly smaller than the reflection bandwidth of the grating. The grating limits the laser to the bandwidth of the grating. Furthermore, the second Fabry-Perot cavity limits the bandwidth to the free spectral range (FSR). The cavity length l is chosen to be sufficiently short so that only one Fabry-Perot mode is supported. The second Fabry-Perot cavity is placed near the center of the first Fabry-Perot cavity to minimize gain except at the desired wavelength where lasing is present. This structure should suppress lasing in the potential unwanted laser cavities that might otherwise exist between grating 110 and reflector 112 and between reflector 114 and mirror 116. The optimal placement of the second Fabry-Perot cavity is determined by the relative lasing thresholds of the competing laser cavities between grating 110 and reflector 112 and between reflector 114 and mirror 116. If the reflectivities of grating 110 and mirror 116 are equal, then the optical placement of the second Fabry-Perot interferometer will be centered in the first Fabry-Perot cavity. If grating 110 has a higher reflectivity than mirror 116, then the second Fabry-Perot interferometer should be placed closer to grating 110 to minimize the gain of the cavity between grating 110 and reflector 112. Conversely, if mirror 116 has the higher reflectivity, the Fabry-Perot interferometer would be placed closer to it.

To suppress lasing in two polarization modes, lasing in one of the two linear polarizations can be suppressed by placing a polarization filter 120 near the center of the optical cavity. The structure of Fig. 8 can be implemented in a doped optical fiber or a silica waveguide.

It should now be apparent that the present invention provides a rare earth doped laser for providing an optical carrier having at least one suppressed mode. The laser may be fabricated with a doped optical fiber or implemented in any other optical transmission medium such that a silicon dioxide substrate is directly doped with the rare earth element. Such a structure is advantageous in that high levels of integration can be achieved.

Although the invention has been described in connection with various exemplary embodiments, it will be apparent to those skilled in the art that numerous adaptations and modifications are possible without departing from the scope of the invention as set forth in the following claims.

Claims (16)

1. A laser for providing an optical carrier, comprising: a cavity resonator (14,L) having longitudinal modes, wherein the cavity resonator (14,L) made of an optical material (12,14) contains a laser medium to provide a laser cavity resonator (110,116); the length (L) of the cavity resonator (14,L) is selected to restrict the longitudinal modes of the laser cavity resonator (14,L), characterized in that a transmission grating (90,130) is present which has a transmission peak in one of the longitudinal modes, and that the grating (90,130) is arranged within the cavity resonator. 2. Laser according to claim 1, wherein the cavity resonator is a Fabry-Perot cavity resonator. 3. Laser according to claim 1 or 2, wherein the cavity resonator (L) comprises a reflector (116) at one end thereof and a reflection grating (110) at the other end thereof. 4. Laser according to claim 2 or 3, wherein the transmission grating (130) is at least approximately centered within the Fabry-Perot cavity (L). 5. Laser according to claim 4, further comprising: a polarization filter (120) in the cavity resonator (L) near the transmission grating. 6. Laser according to one of claims 1 to 5, wherein the optical material is an optical fiber (12, 14) and is at least partially doped with a rare earth element. 7. Laser according to one of claims 1 to 5, wherein the optical material is a waveguide (14) and is at least partially doped with a rare earth element. 8. Laser according to claim 6 or 7, wherein the rare earth element is erbium, neodymium or praseodymium. 9. Laser according to claim 7 or 8, wherein the waveguide is a quartz waveguide. 10. Laser according to claim 9, wherein the transmission grating (130) is in a doped portion of the cavity resonator (L). 11. Laser according to claim 9, wherein the transmission grating (130) is outside a doped portion of the cavity resonator (L). 12. Laser according to one of the preceding claims, wherein: the laser cavity (14) has a ring configuration; and the transmission grating (90) is arranged within a portion of the ring. 13. Laser according to claim 12, wherein the transmission grating (90) is within the doped portion of the ring. 14. Laser according to claim 12, wherein the transmission grating (90) is outside the doped portion of the ring. 15. Laser according to one of claims 12 to 14, further comprising: an optical isolator (16) within the ring. 16. Laser according to one of the preceding claims, wherein the suppression of all but one longitudinal mode of the laser cavity resonator (14,L) is provided.